Make the cookies: Toss the flour, baking soda, and salt into a bowl until combined. Set aside.

In a large bowl using a hand-held mixer or stand mixer with paddle attachment, cream the butter, brown sugar, and white sugar together on medium speed until combined. On low speed, beat in the peanut butter, egg, and vanilla. Turn the mixer to high and beat until light in color and smooth. Turn the mixer to low and slowly pour in the dry ingredients. Mix until combined and a dough is formed.

Cover tightly and chill the cookie dough for at least 1 hour (or up to 3 days). Chilling is mandatory.

Remove cookie dough from the refrigerator. Preheat oven to 350F degrees. Take 1 Tablespoon of dough and roll into a ball. Take another 1 Tablespoon of dough and roll into a ball. Stick a peanut butter cup into one ball, kind of like a "cradle" as shown above. Top the peanut butter cup with other cookie dough ball and seal down the sides so that the peanut butter cup is securely stuffed inside. Roll the large cookie dough ball in your hands to ensure it's smooth. Repeat with the rest of the dough and 18 peanut butter cups.

Bake the cookies for 10-11 minutes or until very lightly browned on the sides. Some cookies may be taller than others, depending how the peanut butter cup was positioned inside. Just press the cookies down a bit to flatten them out. Allow to cool on the cookie sheet for at least 10 minutes and transfer to a wire rack to cool completely.

Make the drizzle: melt chocolate chips and peanut butter together in the microwave, stirring often to prevent seizing. Add 1 teaspoon of shortening if the mixture isn't smooth. Drizzle over cooled cookies.

Cookies stay fresh stored covered at room temperature for up to 7 days. Cookies freeze well, up to 3 months. Stuffed cookie dough balls freeze well, up to 2 months. Bake for 2 extra minutes, do not thaw.